Linear potentiometers



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Nov. 29, 1966 E. F. PIERCE LINEAR POTENTIOMETERS Filed Deo. 5. 1962 United States Patent O 3,289,138 LINEAR POTENTHMETERS Edwin F. Pierce, Pasadena, Calif., assignor to E. and P. Engineering Research and Delevopment Corporation, Reno, Nev., a corporation of Nevada Filed Dec. 3, 1962, Ser. No. 241,674 Claims. (Cl. 338-183) This invention relates generally to potentiometers, and particularly relates to a linear potentiometer of the sliding contact type.

Linear potentiometers of the sliding contact type are well known in the art. The contact pressure between the sliding contacts of such a potentiometer must be carefully balanced. If the potentiometer of the sliding contact type is operated at too low a contact pressure, the contact resistance is relatively high and varies erratically over the life of the instrument. This lcontact resistance may be due to oxidation of the contacts or to corrosion by the atmosphere which may contain sulphur. In view of the low contact pressure, it is not possible to break through such an oxide lm or other lm of chemical compounds which may form on the contact surfaces.

On the other hand, if a high `contact pressure is used with a linear potentiometer of the sliding contact type, wear of the contacting surfaces obviously increases. This wear exists either between the contact element and the resistive element or between two contact elements forming a common return connection. Due to the wear, metal particles or resistive particles may be worn off Iwhich may then be deposited over the resistive element over which a brush or wiper arm slides. Eventually, the resistive element becomes covered with metal particles which create a low resistance path for the electric current rather than the desired high resistance path through the resistive element. Accordingly, the high wear occasioned by high contact pressures in turn limits the contact pressure which can be used with conventional potentiometers.

It is accordingly an object of the present invention to provide an improved linear potentiometer which is not subject to the disadvantages of prior art devices.

It is a further object of the present invention to provide an improved linear potentiometer which permits the use of high Contact pressures and a resultant low contact resistance between the contacting resistive and conductive surfaces.

Another object of the present invention is to provide an improved linear potentiometer which has a low contact resistance between the resistive element and its associated brush and which maintains a uniformly low contact resistance throughout the operating life of the potentiometer.

Still a further object of the present invention is to provide an improved linear potentiometer which has repeatability of the resistance for any particular position of the instrument and which therefore facilitates calibration of the potentiometer.

A linear potentiometer in accordance with the present invention includes a resistive element and a conductive element which serves as a common electric return connection. A conductive brush extends across both the resistive and conductive elements.

Further, in accordance with the present invention, a lubricating material of high dielectric constant is `disposed on the contact surfaces of the two elements and the brush. Any particles which are worn oif either the conductive element or the resistive element or the brush are imbedded or encapsulated by the lubricating material. Due to the high dielectric constant of the lubricating material, these worn off or abraded particles are insulated from each ICC other. In order to adjust the electric resistance of a circuit connected to the resistive and the conductive element, the brush is moved with respect to the two elements to adjust or vary the electrical resistance. The Contact pressure between the brush and the associated conductive and resistive elements should be suiciently high to break through the lubricating film.

Due to the provision of a lubricating material having a high dielectric constant and preferably having a low surface tension, it is possible to increase the contact pressure without creating an undesirable deposit of conductive particles over the resistive element. Thus, it may be desirable to increase the contact pressure to above pounds per square inch (p.s.i.). Such a high contact pressure makes it possible to break through a lm of oxide or other chemical compounds including dirt which may form on the contacting surfaces.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawing, in which:

FIG. l is a sectional view of a linear potentiometer embodying the present invention;

FIG. 2 is a sectional view taken on line 2 2 of FIG. 1;

FIG. 3 is a cross sectional view taken on line 3--3 of FIG. 1;

FIG. 4 is a fragmentary sectional view taken on line 4 4 0f FIG. 3; and

FIG. 5 is a longitudinal sectional view of a modied resistive element for a linear potentiometer in accordance with the present invention.

The linear potentiometer disclosed and claimed herein is also disclosed in a copending application to Edwin F. Pierce entitled Electromechanical Devices. This copending Pierce application is filed concurrently herewith, Serial No. 241,676.

Referring now to the drawing and particularly FIGS. l to 4, there is illustrated a preferred embodiment of a linear potentiometer in accordance with the present invention. The potentiometer includes a housing 10, two end plates 11 and 12, Ia brush holder 13, a brush 14, an adjusting screw 15 which cooperates with a nut 16 and two contact springs 17 and 18. The potentiometer further includes a resistance element 20 and a conductor 21 forming a common return connection.

As clearly shown in FIG. 3, the housing 10 is of generally rectangular cross-section and has elongated cylindrical recesses 22 and 23 through which the resistive element 20 and the conductive element 21 extend. The recesses 22 and 23 are of substantially cylindrical cross section and are open toward the interior of the housing. Rounded projections 24 and 25, of generally semi-circular cross section, may be provided about the circular recesses 22 and 23.

The housing 10 may be made from an insulating plastic material having good electrical properties including a high dielectric constant such, for example, as a polycarbonate. A polycarbonate is a polymer derived from biphenol A or 4,4 dihydroxy diphenyl propane. This compound has the chemical formula Such a polycarbonate is sold in the trade under the name of Lexan by General Electric Company. This plastic material may be readily molded and can be cheaply made in mass production. It is also characterized by great impact strength.

The end plates 11 and 12 consist of insulating material and may, for example, also be made of Lexan. The end plates 11 and 12 may be cemented or otherwise secured to the housing and have an outline corresponding to the cross section of the housing 10. For example, the two end plates 11 and 12 may be cemented to the housing 10, they may be heat-sealed or riveted or connected in any other suitable manner. Lexan, of which the housing 10 and the end plates 11 and 12 are preferably made, have good cementing characteristics and stability. Accordingly, the housing 10 and the end plates 11 and 12 are preferably connected together by applying a solvent to the adjacent edges. Within a few seconds the solvent will dissolve a thin surface layer of the Lexan whereupon the housing 10 and the end plates 11 and 12 may be joined together under pressure and thereafter become firmly cemented.

A central aperture or bore 26 is provided in the end plate 11 for receiving a smooth portion of the adjusting screw 15. The adjusting screw is readily rotatable in the bore 26 of the end plate 11. Furthermore, the end plate 11 is provided with apertures 27 and 28 through which cable conductors 30 and 31 may extend. Similarly, the end plate 12 may be provided with an aperture 32 for a cable conductor 33.

The adjusting screw 15 extends through the bore 26 in the end plate 11. The adjusting screw may have a suitable head 35 with a slot 36 for adjusting the screw, for example, by a screw driver. The adjusting screw 15 may be made of any suitable metal such, for example, as stainless steel which resists corrosion. A split washer 37 secures the screw to the end plate 11. To this end, the screw preferably has a reduced portion or annular slot in which the split washer 27 is mounted. Hence, the adjusting screw 15 is secured between its head 35 and the washer 37, both bearing against the end plate 11. Accordingly, the adjusting screw 15 is readily rotatable for adjustment thereof. The adjusting screw 15 may have a smooth end portion as shown journalled in a bearing 39 projecting from the inner surfaces of the end plate 12.

The brush holder 13 is made of any suitable insulating material and may, for example, consist of Lexan. The brush holder 13 has a rectangular recess 3S open at the top as clearly shown in FIGS. 3 and 4 for receiving the nut 16 which is made of an insulating material such as nylon. The nut 16 is held from rotation in the recess 38. Furthermore, the brush holder 13 is provided with a bore 39 extending centrally therethrough for receiving the adjustment screw 15. The bore 39 is of suflicient diameter to permit free rot-ation of the adjusting screw 15 therein.

The lower portion of the brush holder 13 has two cylindrical recesses 40 for receiving the two contact springs 17 and 18 as shown in FIG. 3. across the lower portion of the brush holder 13 there is a rectangular recess 41 which receives the contact brush 14.

As shown particularly in FIG. 3, the brush 14 is rectangular in side view while its cross section is substantially U-shaped or curved as shown in FIG. 4 to provide a common contact surface both for the common return rod 21 and the resistive rod 20. The brush 14, for example, may consist of an alloy known as nickel silver which includes 18% nickel and 81% copper. However, any conducting material which has a aking action may be used for the brush 14. Alternatively, the brush 14 or its contact surface may be made of or plated with an alloy known as coin silver which consists of 92% silver and 8% copper. It is also feasible to make the brush 14 or its contact surface of palladium, platinum, rhodium or gold. These metals have the property that they are resistant to chemical action land hence the contact surface of the brush 14 remains clean. They also have a aking action which helps to maintain a clean contact surface.

It will be noted that the two contact springs 17 and 18 Furthermore, extending t 4 are symmetrically disposed at the two ends of the brush 14. Accordingly, they exert a uniform pressure pressing the brush 14 against both rods 20 and 21.

The resistive rod 20 may consist of any suitable resistive material such as carbon. As explained before, the rod 20 `is received in the cylindrical recess 22 in the housing body 10. It will be noted that the contact surface of the rod 20 remains uncovered and extends into the interior of the housing.

The conductive rod 21 is received in the similar cylindrical recess 23 and may consist of any conductive material. For example, the conductive rod 21 may consist of copper or brass which has been plated with nickel. Alternatively, the common return rod 21 may be plated with or made of tungsten, molybdenum or stainless steel. ln general, the surfaces of both of the rods 20 and 21 should be porous or pitted for a purpose to be described hereinafter. The above-mentioned materials, such as carbon, nickel-plated metal, tungsten, molybdenum or stainless steel, all have such a pitted surface.

The resistive element or rod 20 may be connected in any suitable manner on both ends to a cable conductor 30 or 33 as shown. Thus, either the cable conductor 30 or the cable conductor 33 may be connected to an electric circuit. The return connection may be made through the cable conductor 31 which is electrically connected in any conventional manner to the return conductor 21.

In accordance with the present invention, a lubricating material of high dielectric constant is spread over the exposed surface of the common return rod 21 as well as over the exposed surface of the resistive rod 20 and the outer surface of the brush 14. This lubricating material should preferably also have a low surface tension or in other words, a surface anity for the contacts such as the rods 20 and 21. Preferably, use is made of the capillary action of the pits or depressions of the nickel plating of the common return bar 21. Similarly, the carbon of the resistive rod 20 has pits or depressions which trap the lubricating material. In addition, the lubricating material or dielectric fluid should maintain its characteristics over a normal ambient temperature range which may extend from below room temperature to at least -}160 F.

By way of example, the insulating lubricating material which is used in accordance with the present invention may include a major proportion of mineral oil of the type used for high voltage transformers. This mineral or transformer oil withstands high temperatures as well as low temperatures and. has a high dielectric constant because it is used for transformers carrying up to 300,000 volts. This mineral oil or transformer oil may be emulsied and may have a base of a tallow grease which will not carbonize out under conditions of wear. A mineral detergent may be added to increase the wetting action or to decrease the surface tension.

An insulating lubricant of this type is sold in the trade under No. 871 Switch Lube by Lubrication Company of America. This insulating lubricant includes at least 93% mineral oil and is of relatively high viscosity. Thus, at F., the viscosity may be 2,600 centistokes per second. This mineral oil has a Hash point at 550 F. In other words, when the oil is heated to 550 F., it will ash under the inuence of an electric arc. Thus, the lubricant will withstand arcing up to 550 F. It may further be added that No. 871 Switch Lube shows substantially no corrosion.

The metal particles worn off primarily from the brush 14 as well as particles worn off from the carbon rod 20 and the conductive rod 21 are imbedded in the insulating lubricant which is used to lubricate the contact surfaces. Since the lubricating material used is accordance with the present invention has a high dielectric constant, the worn off metal particles or carbon particles are insulated from each other and cannot cause undesirable short circuits by plating, for example, on the carbon rod 20. In other words, the worn olf metal particles are imbedded or encapsulated within a lrn that is thick enough so that individual metal particles are insulated from each other. Thus, even if these metal particles are arranged in a chain, they should be insulated from each other so that they will not conduct current. On the other hand, due to the high dielectric constant of the lubricating material, it is necessary that the contact pressure be large enough to break through the lm of lubricating material. Thus, the contact pressure should be high enough to break through this lubricating lm. The contact pressure required for' this will vary with the particular materials used for the cooperating contact surfaces and with the properties of the insulating and lubricating lm. However, it has been found that if the contact pressure is too low, the contact resistance increases and erratic operation of the circuit takes place.

On the other hand, a high contact pressure is sometimes desirable because such a high contact pressure will scrape off or wear off any film of oxidation or other impurities or dirt particles which may form on the contacting metals.

Accordingly, at the present time it is preferred to utilize a Contact pressure of at least 100 pounds per square inch or high-er'. As the contact areas wear, the contact areas increase, and hence the pressure in pounds per square inch decreases from the original 800 pounds per square inch and may reduce to about 400 pounds per square inch at the end of the useful life of the device. However, it should be noted that the potentiometer of the invention can be operated at lower pressures, that is, at pressures below 100 pounds per square inch.

The potentiometer illustrated in FIGS. 1 through 4, may be assembled in the following manner:

At rst, the metal rod 21 and the carbon rod 20 are inserted into their respective recesses 23 and 22. Then the adjusting screw 15 is inserted through the bore 26 land the end plate 11 and is locked in place by the split washer 37. Thereafter, the two contact springs 17 and 18 are inserted into their respective recesses 40 and the brush 14 is put into its recess 41. Thereafter, the nut 16 is dropped int-o its reoess 38. The brush holder 13 which is now completely assembled may be put on the adjusting crew 15 by rotating the screw head and holding the nut 16 against rotation of the screw. Care should be taken that the :brush 14 is lifted so that it rides over the rods 20 and 21. The potentiometer is now essentially assembled -and all that needs to be done is to cement or otherwise secure the end plates 11 and 12 to the housing portion 10. The cable conductors 30, 31 and 33 may be connected to the two rods 20 and 21 in any suitable manner.

The operation of the linear potentiometer of FIGS. 1 to 4 will now be explained. An electric circuit may be connected between the cable conductors 30 and 31. In that case, the resist-ance of the resistive rod 20 between the cable conductor 30 and the brush 14 is in the electrical circuit. The electric current then travels from the cable conductor 30 through the resistive rod 20 and the brush 14 into the common return connection 20 and out through the cable conductor 36. Alternatively, it is also feasible to connect the circuit between the cable conductor 33 and the conductor 31. In that case, the electric resistance of the resistive rod 20 Ibetween cable conductor 33 and :brush 14 is connected in the electric-al circuit.

It will be noted that the adjusting screw 15 forms no part of the electrical circuit. The screw 15 is insulated from the electric circuit through the end plate 11 and through the brush holder 13 and the insulating nut 16. This arrangement has the advantage that any variable resist-ance which may otherwise be created between the brush and the adjusting screw 15 is eliminated.

Due to the use of a lubricating material of high dielectric constant, the Contact resistance remains low throughout the useful life of the potentiometer. Furthermore, the lubricant protects the surfaces of the brush 14, the metal rod 21 and the resistive rod 20 from corrosion,

oxidation and other chemical action which might change their electric resistance. If impurities or an oxide film should form on the contacting surfaces, the contact pressure may be made high enough to break through such a lm. Any worn off partciles -of metal or carbon, for example, are imbedded in the lubricant which may be No. 871 Switch Lube.

The electric resistance in the circuit is adjusted, for example, by a screw driver inserted into the slot 36 of the head 35 of the adjusting screw. Alternatively, the adjusting screw 1S may be driven through an electric motor or may be rotated in any other conventional manner.

A modification of the resistive rod 20 is illustrated in FIG. 5. Here, there is provided a rod or cylinder 45 of insulating material having a helical groove 46 of substantially circular cross section, and a resistive wire 47 is inserted into the helical groove 46 in such a manner that the surface of the wire 47 is substantially ush with the surface 48 of the cylinder 45. The lubricating material is retained in the gap between the wire 47 and helical recess 46. Accordingly, the brush 14 slides between each portion of the wire 47 and the insulating surface 48. Preferably, the wires 47 are so close together that the conductive surface of the brush 14 always bridges two yadjacent wires 47.

There has thus been disclosed an improved linear potentiometer having sliding contacts. The potentiometer of the invention has a low contact resistance which remains uniformly low throughout the operating life of the instrument. The construction of the potentiometer is simplied by the adjustment screw 15 and its nut 16. Furthermore, the housing 10, the end plates 11 and 12 and the brush holder 13 may be molded thus reducing the costs of the potentiometer. The adjustment screw 15 is insulated from the electric circuit which promotes repeatablity of the resistance for any particular position of the brush holder 13. Accordingly, the potentiometer can readily be calibrated and will not change its resistance throughout its useful life. Additionally, the resistance for each position of the brush holder will always remain the same.

The invention and its attendant advantages will be understood from the foregoing description. It will be apparent that various changes may `be made in the form, construction and arrangement of the parts of the invention without departing from the spirit and scope thereof or sacricing its material advantages, the arrangement hereinbefore described being merely by way of example. I do not Wish to be restricted to the specic form shown or uses rmentioned except as defined in the accompanying claims, wherein various portions have been separated for clarity of reading and not for emphasis.

I claim:

1. A linear potentiometer comprising:

(a) an elongated housing of insulating material having two substantially parallel spaced recesses therein of substantially circular cross section and open toward the interior of said housing;

(b) a conductive rod shaped element disposed in one of said recesses of said housing to expose a contact surface;

(c) a resistive rod shaped element disposed in the other one of said housing portions to expose a contact surface;

(d) a brush hol-der of insulating material movably disposed in said housing for movement parallel to the elongate extents of said conductive and resistive elements and having an elongated first recess therein facing said elements;

(e) an elongated brush of conductive material disposed in the first recess of said brush holder;

(f) two coil springs disposed in said brush holder and urging each end of said brush into electric contact with a respective one of said elements;

essaies (g) an adjusting screw exten-ding through said housing parallel to said conductive and resistive elements and rotatable therein and further extending through -said brush holder, said brush holder having a second recess;

(h) a nut of insulating material in threaded engagement with said screw and disposed in said second recess of said brush holder, said second recess in said brush holder being so shaped as to prevent rotation of said nut, whereby rotation of said screw will adjust the relative position of said brush holder and brush with respect to said elements to adjust the electric resistance between said conductive element and said resistive element; and

(i) terminal means for each of said conductive and resistive elements connected to the elements through the housing.

2. A linear potentiometer comprising:

(a) an elongated housing of insulating material having two substantially parallel spaced recesses therein of substantially circular cross section and open toward the interior of said housing;

(b) a conductive rod shaped element disposed in one of said recesses of said housing to expose a contact surface, said conductive element having a pitted surface;

(c) a resistive rod shaped element disposed in the other one of said housing portions to expose a contact surface, said resistive element having a porous surface;

(d) a brush holder of insulating material movably disposed in said housing for movement parallel to the elongate extents of the conductive and resistive elements and having an elongated first recess therein facing said elements;

(e) an elongated brush of conductive material disposed in the first recess of said brush holder, said brush having a aking contact surface;

(f) an insulating lubricating material disposed on the contact surfaces of said elements and brush, the lubricating material having a surface tension sufficiently low that it wets the contact surfaces and fills the pores and pits in the contact surfaces of said elements;

(g) two coil springs disposed in said brush holder and urging each end of said brush into electric contact with a respective one of said elements with a contact pressure sufficient to break through a film of the lubricating material on said brush and elements;

(h) an adjusting screw extending through said housing parallel to said elements and rotatable therein and further extending through said brush holder, said brush holder having a second recess;

(i) a nut of insulating material in threaded engagement with lsaid screw and disposed in said second recess of said brush holder, said second recess in said brush holder being so shape-d as to prevent rotation of said nut, whereby rotation of said screw will adjust the relative position of said brush holder and brush with respect to said elements to adjust the electric resistance between said elements; and

(j) terminal means for each of said conductive and resistive elements connected to the elements through the housing.

.3. A linear potentiometer comprising:

(a) an elongated housing of insulating material having two substantially parallel spaced recesses therein of substantially circular cross section and open toward the interior of said housing;

(b) an end plate of insulating material connected to each end of said housing for providing an enclosed space;

(c) a conductive rod shaped element disposed in one of said recesses of said housing to expose a contact surface;

(d) a resistive rod shaped element disposed in the other one of said housing portions to expose a contact surface;

(e) a brush holder of insulating material movably disposed in sai-d housing for movement parallel to the elongate extents of said elements and having an elongated rst recess therein facing said elements;

(f) an elongated brush of conductive material disposed in the first recess of said brush holder;

(g) two coil springs disposed in said brush holder and urging each end of said brush into electric contact with a respective one of said elements;

(h) an adjusting screw extending through one of said end plates parallel to said elements and rotatable therein and further extending through said brush holder, said brush holder having a second recess;

(i) a nut of insulating material in threaded engagement with said screw and disposed in said second recess of sai-d brush holder, said second recess in said brush holder being so shaped as to prevent rotation of said nut, whereby rotation of said screw will adjust the relative position of said brush holder and brush with respect to said elements to adjust the electric resistance between said elements; land (j) terminal means for each of said conductive and resistive elements connected to the elements through the housing.

4. A linear potentiometer comprising:

(a) an elongated housing of insulating material having two substantially parallel spaced recesses therein of substantially circular cross section and open toward the interior of said housing;

(b) an end plate of insulating material connected to each end of said housing for providing an enclosed space to which said recesses open;

(c) a conductive rod shaped element disposed in one of said recesses of said housing to expose a contact surface;

(d) a resistive rod shaped element disposed in the other one of said housing portions to expose a contact surface;

(e) a conductor connected to one end of each of said elements, each of said conductors extending through one of said end plates;

(f) a brush holder of insulating material movably disposed in said housing for movement parallel to the elongate extents .of said elements and having an elongated first recess therein facing said elements;

(g) an elongated brush of conductive material disposed in the first recess of said brush holder;

(h) two coil springs dispose-d in said brush holder and urging each end of said brush into electric contact with a respective one of said elements;

(i) an adjusting screw extending through one of said end plates parallel to said elements and rotatable therein and further extending through said brush holder, sai-d brush holder having a second recess; and

(j) a nut of insulating material in threaded engagement with said screw and disposed in said second recess of said brush holder, said second recess in said brush holder being so shaped as to prevent rotation of said nut, whereby rotation of said screw will adjust the relative position of said brush holder and brush with respect to said elements to adjust the electric resistance between said conductors.

5. A linear potentiometer for varying the resistance of an electric circuit connected thereto comprising:

(a) a housing defining therein an elongate internal chamber;

(b) an elongate resistive element having a porous surface disposed in the chamber;

(c) an elongate conductive element having a porous surface disposed in the chamber in spaced parallel relation to the resistive element;

(d) a brush of conductive material disposed in the (h) screw means extending along the chamber parallel housing and having a contact surface defined by a to the resistive and conductive elements and accesyCOIldllCtV material Which fiakeS When abraded; sible externally of the housing for rotation thereof,

(e) a brush holder disposed in the chamber adjacent the screw means cooperating with the brush holder the resistive and conductive elements and mounted 5 for moving the holder and the brush along Said for movement in the Chamber Parallel te the elongate elements in response to rotation of the screw means; extents of said elements, the brush holder receiving and the brush and positioning the brush transversely of said elements;

(f) a quantity of electrically insulating lubricant dis- 10 posed on said elements and on the brush contact surface for encapsulating conductive particles worn from the brush contact surface, .the lubricant having a dielectric constant suiiiciently high to effectively (i) terminal means connected to said resistive and conductive elements internally of the housing and accessible externally of the housing for connecting the potentiometer into an electric circuit.

References Cited by the Examiner insulate the particles from each other and having a 15 UNITED STATES PATENTS surface tension sufciently low that the lubricant Wets 1 926 196 9/1933 Dacosta 338 132 the surfaces of the elements and lis the pores 2,779827 1/1957 Brown t "3 38 194X thereof;

(g) spring means cooperating between the brush and lilms et al the brush holder for biasing the brush contact sur- 20 lott face into physical and conductive contact with the 2914744 11/1959 schendel 338-164 resistive and conductive elements with a Contact 219351716 5/1960 Bguffls 338-180 pressure lsufficient to cause the brush contact surface 3,050,704 8/1962 Dlckmson et al' 338-180 to be abraded in response to movement of the brush along said elements; 25 ANTHONY BARTIS, Primary Examiner. 

1. A LINEAR POTENTIOMETER COMPRISING: (A) AN ELONGATED HOUSING OF INSULATING MATERIAL HAVING TWO SUBSTANTIALLY PARALLEL SPACED RECESSES THEREIN OF SUBSTANTIALLY CIRCULAR CROSS SECTION AND OPEN TOWARD THE INTERIOR OF SAID HOUSING; (B) A CONDUCTIVE ROD SHAPED ELEMENT DISPOSED IN ONE OF SAID RECESSES OF SAID HOUSING TO EXPOSE A CONTACT SURFACE; (D) A BRUSH HOLDER OF INSULATING ELEMENT DISPOSED IN THE OTHER ONE OF SAID HOUSING PORTIONS TO EXPOSE A CONTACT SURFACE; (D) A BRUSH HOLDER OF INSULATING MATERIAL MOVABLY DISPOSED IN SAID HOUSING FOR MOVEMENT PARALLEL TO THE ELONGATE EXTENTS OF SAID CONDUCTIVE AND RESISTIVE ELEMENTS AND HAVING AN ELONGATED FIRST RECESS THEREIN FACING SAID ELEMENTS; (E) AND ELONGATED BRUSH OF CONDUCTIVE MATERIAL DISPOSED IN THE FIRST RECESS OF SAID BRUSH HOLDER; (F) TWO COIL SPRINGS DISPOSED IN SAID BRUSH HOLDER AND URGING EACH END OF SAID BRUSH INTO ELECTRIC CONTACT WITH A RESPECTIVE ONE OF SAID ELEMENTS; (G) AN ADJUSTING SCREW EXTENDING THROUGH SAID HOUSING PARALLEL TO SAID CONDUCTIVE AND RESISTIVE ELEMENTS AND ROTATABLE THEREIN AND FURTHER EXTENDING THROUGH SAID BRUSH HOLDER, SAID BRUSH HOLDER HAVING A SECOND RECESS; (H) A NUT OF INSLATING MATERIAL IN THREADED ENGAGEMENT WITH SAID SCREW AND DISPOSED IN SAID SECOND RECESS OF SAID BRUSH HOLDER, SAID SECOND RECESS IN SAID BRUSH HOLDER BEING SO SHAPED AS TO PREVENT ROTATION OF SAID NUT, WHEREBY ROTATION OF SAID SCREW WILL ADJUST THE RELATIVE POSITION OF SAID BRUSH HOLDER AND BRUSH WITH RESPECT TO SAID ELEMENTS TO ADJUST THE ELECTRIC RESISTANCE BETWEEN SAID CONDUCTIVE ELEMENT AND SAID RESISTIVE ELEMENT; AND (I) TERMINAL MEANS FOR EACH OF SAID CONDUCTIVE AND RESISTIVE ELEMENTS CONNECTED TO THE ELEMENTS THROUGH THE HOUSING. 